Remove WebRuntimeFeatures::enableSpeechInput

Depends on Chromium r268646.

BUG=223198

Review URL: https://codereview.chromium.org/269953008

git-svn-id: svn://svn.chromium.org/blink/trunk@173543 bbb929c8-8fbe-4397-9dbb-9b2b20218538